~ ~ ~ 6&i 1 JPL PUBLICATION 86-18 a- /.N -2P-d Science Oppofiunities From the Topex/Poseidon Mission R. Stewart L.-L. FU I Jet Propulsion Laboratory I M. Lefebvre Centre Spatial de Toulouse (NASA-CR -1 797 52) SCIENCE CPFOFiTUN ITXES PROE N87-10671 IHE POPEX/POSEICCP M1:ESXCN (Jet Fropulsion Lac.) 6E p CSCL 08C Unclas I G3/48 ri4225 July 15, 1986 runsn National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology Pasadena, California - JPL PUBLICATION 86-18 Science Opportunities From the TopexlPoseidon Mission R. Stewart L.-L. FU Jet Propulsion Laboratory M. Lefebvre Centre Spatial de Toulouse July 15, 1986 NASA National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology Pasadena, California The research described in this publication was carried out in part by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aero- nautics and Space Administration. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not constitute or imply its endorsement by the United States Government or the Jet Propulsion Laboratory, California Institute of Technology. National Aeronautics and Space Administration Washington, D.C. 20546 4ttn of EE Dear Colleague: The first oceanographic instrument in space was a microwave sensor that flew aboard NASA's Skylab in 1972. Since that time, ocean-related sensors have flown aboard several U.S. I satellites, including Nimbus-7 and Seasat--the first dedicated oceanographic satellite--launched I in 1978. The NASA Seasat altimeter clearly demonstrated that spaceborne microwave ! altimeters were capable of obtaining global measurements of ocean topography with sufficient I precision and accuracy to be useful for oceanographic studies. I Looking to the future, both NASA and the French Centre National d'Etudes Spatiales (CNES), in cooperation with other national partners, have been planning the coordinated TOPEWPOSEIDON dedicated altimeter satellite mission for observing the oceans. In concert I with the TOPEX/POSEIDON mission, NASA also has plans for a series of spaceflight activities designed to further our understanding of ocean circulation, air-sea interactions, and I the biological processes taking place in the upper ocean. This series--the NASA Scatterometer (NSCAT) to fly aboard the U.S. Navy Remote Ocean Sensing System (N-ROSS) satellite, the NASA/CNES TOPEX/POSEIDON altimeter mission, and a color scanner for a platform of opportunity--will also contribute to the Global Change, the International Geosphere/Biosphere, I I and World Climate Research Programs. I would like to take this opportunity to inform you that the TOPEX/POSEJDON mission has been proposed by NASA for a new start in FY 87. The TOPEX/POSEIDON measurements of surface topography of the global oceans have the potential to enhance our Understanding of the oceanic circulation, as well as the ocean's role in climate variability. In addition, these I I measurements will provide information that will allow studies of tides, sea state, and marine geophysics. This Science Opportunities Document describes the capabilities of the TOPEX/POSEIDON i instruments and outlines various types of experiments that could be performed utilizing TOPEX/POSEIDON data. I hope that this document will aid you and your colleagues in preparing to exploit the potential offered by the jointly sponsored NASAKNES TOPEX/POSEIDON mission, and I invite you to share in the scientific planning and execution of this exciting project. Sincerely, , S. G. Tilford, Director , Earth Science and Applications Division I I Office of Space Science and Applicatio~s I Abstract The U.S. National Aeronautics and Space Administration (NASA) and the French Centre National d’Etudes Spatiales (CNES) propose to conduct a Topex/Poseidon mission for studying the global ocean circulation from space. The mission will use the techniques of satellite altimetry to make precise and accurate measurements of sea level for several years. The measurements will then be used by Principal Investigators (selected by NASA and CNES) and by the wider oceanographic community working closely with large inter- national programs for observing the Earth, on studies leading to an improved under- standing of global ocean dynamics and the interaction of the ocean with other processes influencing life on Earth. The major elements of the mission include a satellite carrying an altimetric system for measuring the height of the satellite above the sea surface; a precision orbit determination system for referring the altimetric measurements to geodetic coordinates; a data analysis and distribution system for processing the satellite data, verifying their accuracy, and making them available to the scientific community; and a Principal Investigator program for scientific studies based on the satellite observations. This document describes first of all the satellite, its sensors, its orbit, the data analysis system, and plans for verification and distribution of the data. It then discusses the expected accuracy of the satellite’s measurements and their usefulness to oceanographic, geophysical, and other scientific studies. Finally, it outlines the relationship of the Topex/Poseidon mission to other large programs, including the World Climate Research Program, the U.S. Navy’s Remote Ocean Sensing System satellite program, and the Euro- pean Space Agency’s ERS-1 satellite program. V Acknowledgment A substantial portion of Section IV on the ocean topography experiment was adapted from the report Satellite Altimetric Measurements of the Ocean: Report of the TOPEX Science Working Group, a group chaired by Professor Carl Wunsch. vi . Contents 1. Introduction .................................................. 1 II. Topex/Poseidon Mission ...................................... 5 A . Requirements and Capabilities .............................. 5 1 . Satellite ................................................ 6 2. Instruments ............................................. 6 3. Accuracy of Sea-Level Measurements .......................... 7 4 . Sampling Strategy ........................................ 7 5. Tidal Aliases ............................................ 7 6 . Coverage and Duration ..................................... 7 7. Orbit Maintenance ........................................ 8 8. Phases of the Mission ...................................... 8 9. Data Reduction and Distribution ............................... 9 10. Retrievability ............................................ 10 B. Sensors .................................................. 10 1 . NASA Dual-Frequency Altimeter ............................... 10 2. CNES Solid-state Altimeter .................................. 11 3. Topex Microwave Radiometer ................................ 11 4 . Laser Retroreflector Array ................................... 12 5. Tranet Beacon ........................................... 12 6 . Doris Receiver ........................................... 13 7. Global-Positioning-System Demonstration Receiver ................. 14 C. Orbit ..................................................... 14 D. Data Management and Verification .......................... 16 1 . Satellite Data ............................................ 16 2. Historical Data ........................................... 19 3. Verification of Measurements ................................. 19 111. Measurement Accuracy ....................................... 23 A . Orbit Errors ............................................... 23 B. Altimeter Errors ........................................... 25 C . Errors Due to the Environment .............................. 27 1 . The Ionosphere .......................................... 27 2. TheTroposphere ......................................... 27 3. Oceanwaves ............................................ 27 4 . Rain .................................................. 28 D. Geoid Uncertainty ......................................... 28 E. Tides .................................................... 30 F. The Inverted Barometer .................................... 30 IV. Topex/Poseidon: The Ocean Topography Experiment ........... 31 A . Ocean Circulation Experiments ............................. 31 1 . Geostrophic Currents and Topography .......................... 32 2. Deviations from Geostrophy .................................. 33 3. Oceanic Variability ........................................ 35 4 . Measuring the Variable Ocean Circulation ........................ 36 5. Measuring the Permanent Ocean Circulation ...................... 38 6 . TheOceanat Depth ....................................... 40 vii B. Geophysical Experiments .................................. 41 1. Oceanic Bathymetry ....................................... 41 2 . Rigidity of the Lithosphere ................................... 42 3. Mantle Convection ........................................ 42 4 . Geodesy ............................................... 42 C . Auxiliary Experiments ...................................... 42 1. Calculation of Geocentric Oceanic Tides ......................... 43 2 . Forecasting of Ocean Waves and Storm Surges ................... 44 3. Interaction of Waves and Currents ............................. 44 4 . Ocean Currents and the Earth's Angular Momentum Budget ........... 44 5 . Fisheries ..............................................